The Jaffe reaction (picrate) was studied to elucidate its molecular and spectral mechanism, specifically with regard to interfering substances. 1. General: The interference caused by cepha antibiotic was investigated. All cepa antibodies and benzyl-penicillin were found to react with alkaline picrate, contrary to previous reports but they were classed according to their probability of causing interference. 2. Blank correction factor: A correction factor was derived for those spectrophotometric reactions where an absorbing reagent is consumed during the reaction. 3. The drugs phenacemide and cephalothin cause interfere negatively with the Du Pont ACA but positively with the ASTRA and SMAC, because their reaction with picrate is over when the timing interval begins and they have lost absorbance when the timing interval ends. 4. Spectra of Picric acid: The spectral pattern of picric acid is dependent on the solvent. Thoroughly dried picric acid shows a peak in water which is absent in CC14. The significance of the project lies in our ability to improve the selectivity and decrease cost of determining creatinine with picrate.